Next, we consider the HHG spectra as a function of pump wave intensity. In
Figs. 4(c) and 4(d), we show the HHG spectra as a function of field
amplitude and the harmonic order for a fixed frequency. The HHG spectra have
interesting structures. First of all, it is clearly seen excitonic lines
(marked by the white horizontal lines). These excitonic lines define the
borders of the plateaus and within each plateau, the cutoff harmonic
linearly increases with increasing the field strength. Then, reaching the
harmonic \(\sim \) 160 which corresponds to the transition of the lowest
occupied molecular orbital to the highest unoccupied molecular orbital, the
HHG rate is saturated. Note that linear dependence of the cutoff harmonics
on the field strength is inherent to HHG via discrete levels [1]},
or in crystals with linear energy dispersion [2]}, [3]}, [4]}.
